1809 in science
- Carl Friedrich Gauss publishes Theoria motus corporum coelestium in sectionibus conicis solem ambientum in Hamburg, introducing the Gaussian gravitational constant and containing an influential treatment of the least squares method.
- S. D. Poisson publishes Sur les inégalités séculaires des moyens mouvements des planètes and Sur la variation des constantes arbitraires dans les questions de mécanique in the Journal of the École Polytechnique, extending Lagrange's theory of planetary orbits.
- Jean-Baptiste Lamarck publishes Philosophie Zoologique, outlining his theory of evolution.
- Johann Heinrich Friedrich Link first describes Penicillium.
- William Maclure publishes the first geological map of the United States with accompanying memoir.
- December 25 – American physician Ephraim McDowell performs the world's first ovariotomy, the removal of an ovarian tumor.
- Philippe Pinel publishes accounts of what would later be regarded as schizophrenia.
- February 11 – Robert Fulton patents the steamboat in the United States.
- May 5 – Mary Kies becomes one of the first women granted a U.S. patent, for a technique of weaving straw hats with silk and thread.
- Samuel Thomas von Sömmerring invents a water voltameter electrical telegraph.
- William Hyde Wollaston invents the reflecting goniometer.
- January 4 – Louis Braille (died 1852), inventor.
- February 12 – Charles Darwin (died 1882), naturalist.
- February 15 – Cyrus McCormick (died 1884), inventor.
- February 21 – Carl Ernst Bock (died 1874), physician and anatomist.
- April 7 – James Glaisher (died 1903), meteorologist and balloonist.
- April 15 – Hermann Günther Grassmann (died 1877), mathematician.
- April 20 – James David Forbes (died 1868), physicist, glaciologist and seismologist.
- August 29 – Oliver Wendell Holmes, Sr. (died 1894), physician and writer.
- November 22 – Bénédict Morel (died 1873), psychiatrist.
- Date unknown
- May 17 – Leopold Auenbrugger (born 1722), Austrian physician.
- August 18 – Matthew Boulton (born 1728), English mechanical engineer.
- October 11 – Meriwether Lewis (born 1774), American explorer.
- December 16 – Antoine François, comte de Fourcroy (born 1755), French chemist.
- December 29 – Thomas Barker (born 1722), English meteorologist.
- Grun, Bernard (1991). "1809". The Timetables of History (3rd ed.). New York: Simon & Schuster. p. 379. ISBN 0-671-74919-6.
- Link, J. H. F. (1809). "Observationes in ordines plantarum naturales. Dissertatio I". Magazin der Gesellschaft Naturforschenden Freunde Berlin (in Latin). 3: 3–42.
- "Observations on the Geology of the United States explanatory of a Geological Map". Transactions of the American Philosophical Society. 4: 91. 1809.
- Othersen, H. Biemann (May 2004). "Ephraim McDowell The Qualities of a Good Surgeon". Annals of Surgery. 239 (5): 648–650. doi:10.1097/01.sla.0000124382.04128.5a. PMC . PMID 15082968.
- Heinrichs, R. W. (2003). "Historical origins of schizophrenia: two early madmen and their illness". Journal of the History of the Behavioral Sciences. 39: 349–63. doi:10.1002/jhbs.10152. PMID 14601041.
- "Robert Fulton patented the steamboat in 1809". Thinkfinity. Verizon. Retrieved 2011-04-15.
- "The Fulton Patents". Today in Science History. Retrieved 2011-04-15.
- "Mary Kies - Patenting Pioneer". About.com. Retrieved 2007-05-14.